A 3G-enabled iPhone and the next generation of the OS X operating system are among the expected announcements for Apple's Worldwide Developers Conference (WWDC), which takes place in San Francisco this week.

The annual five-day event targets hardware and software programmers using Apple's software and devices, but is also used to showcase new developments by the company. An updated iPhone handset was the most popular expectation among online forums.

The current model of the popular device is listed as unavailable on both the US and UK Apple Store sites, possibly indicating that the path is being cleared for new stock. And demand for 3G functionality has been high for some time, said Analysys analyst Mark Heath.

"Apple has been suggesting that the stock-outs are due to unexpected demand, rather than the impending release of a new handset," he said.

"However, it will be very disappointing if Apple doesn't launch in June, because the longer it takes to get a 3G model out, the more advantage is given to competitors such as Nokia. Without this, the iPhone is missing something that other handsets have, so the sooner Apple can get it out the better."

Aside from a new iPhone, the next instalment of Apple's OS X operating software is a strong possibility on the list of announcements. At the delayed launch of OS 10.5 Leopard last October, chief executive Steve Jobs told the New York Times that he wished to stick to a deployment cycle of 12 to 18 months.

A member of the online forum MacRumors claims to have found a reference to " OS 10.6" within the code of the most recent iPhone software development kit. And recent activity from Apple's main rival further increases the likelihood of a new revelation, said Jupiter analyst Ian Fogg.

"Microsoft mentioned the next version of Windows a few weeks ago, and Leopard has now been out since last autumn. It would be natural for Apple to start talking about a new operating system now, especially given the audience next week."

But the biggest news from WWDC could be something no one has seen coming, added Fogg.

"Apple likes announcements that include surprises, so my expectation is that there will be a least one thing that no one has guessed. Expect the unexpected. "